Note 10 - Stockholders' Equity - Summary of Warrant Activity (Details) - Purchase Warrants [Member]	9 Months Ended
	Sep. 30, 2021 $ / shares shares
Shares purchasable under outstanding warrants, beginning balance (in shares) | shares	3,206,910
Shares purchasable, average price per share, beginning balance (in dollars per share) | $ / shares	$ 6.36
Stock purchase warrants issued (in shares) | shares	0
Stock purchase warrants issued (in dollars per share) | $ / shares	$ 0
Stock purchase warrants exercised (in shares) | shares	(1,815,012)
Stock purchase warrants exercised (in dollars per share) | $ / shares	$ 6.64
Stock purchase warrants expired (in shares) | shares	(231)
Stock purchase warrants expired (in dollars per share) | $ / shares	$ 7.58
Shares purchasable under outstanding warrants, ending balance (in shares) | shares	1,391,667
Shares purchasable, average price per share, ending balance (in dollars per share) | $ / shares	$ 6.00